1. Technical Field
The present invention relates to an optical encoder and, more particularly to a lens design and arrangement suitable for use in a compact “off axis” readhead of an optical encoder wherein a principal illumination ray is offset from the lens axis in a direction perpendicular to the reading direction of the encoder.
2. Description of the Related Art
In the related art, there is known an optical encoder including a scale having latticed or striped scale markings and a readhead. A readhead has a light source that emits light to the scale and a light receiving element that receives the light reflected by the scale, so that the position of the readhead with respect to the scale is measured based on the light received by the light receiving element. Some sample optical encoders are disclosed in Japanese Patent Application Publication Nos. 2003-307440 (“Patent Document 1”) and 2006-284564 (“Patent Document 2”).
The optical encoder disclosed in Patent Document 1 includes scale markings (a scale) and a reflection type optical reader (a readhead) having a light-emitting diode (a light source), an object lens, and a light receiving unit (a light receiving element). In addition, the reflection type optical reader has a half-mirror arranged between the light-emitting diode and the object lens so that an optical path of the light emitted from the light-emitting diode and directed toward the scale markings through the object lens is separated from an optical path of the light reflected by the scale markings and directed toward the light receiving unit through the object lens.
However, in the optical encoder disclosed in Patent Document 1, as the light receiving unit receives the light through the half-mirror, there may be a problem that a sufficient amount of light cannot be obtained. In addition, if the amount of the light emitted from the light-emitting diode increases in order to obtain a sufficient amount of light, power consumption of the light-emitting diode increases, and further, lifetime of the light-emitting diode decreases.
On the other hand, the photoelectric encoder (optical encoder) disclosed in Patent Document 2 includes a scale, a light source, a lens, and a light receiving element. In the photoelectric encoder, an optical path of the light emitted from the light source and directed toward the scale is separated from an optical path of the light reflected by the scale and directed toward the light receiving element through the lens, and a sufficient amount of light is obtained by the light receiving element by arranging the scale, the lens, and the light receiving element in a Scheimpflug relationship.